1. Field of the Invention
This invention relates to a display data driving integrated circuit (IC) for driving a matrix display unit, and more particularly to a control circuit for controlling the operation timing relationship between the groups of display driving signal output driving buffer circuits.
2. Description of the Related Art
A display data driving IC for driving a matrix display unit (e.g., a liquid-crystal unit, EL (electroluminescence) display unit, a fluorescent display unit, or a DC plasma display unit) by a dynamic technique basically performs serial/parallel conversion of serially inputted display data signals, and outputs the parallel display data to the outside via the groups of display driving signal output driving buffer circuits and the groups of output terminals.
FIG. 1 shows an example of the display driving signal output driving buffer circuit section in a conventional display data driving IC.
Numerals 71.sub.1 to 71.sub.n indicate a plurality of buffer circuits (e.g., as many buffers as suffices for 64 bits or 64 channels); 72.sub.1 to 72.sub.n represent output terminals provided so as to correspond to the driving buffer circuits 71.sub.1 to 71.sub.n ; 73 denotes a single operation control signal input terminal to which an operation control signal IN is supplied; and 741 and 742 indicate inverter circuits for waveform-shaping the input signal for the control terminal 73 and supplying it to a single operation control signal line 75.
The driving buffer circuits 71.sub.1 to 71.sub.n are supplied with the individual bit signals of 64 bits of parallel display data, respectively, from a serial/parallel converter circuit 76, and are all supplied with the operation control signal from the operation control signal line 75.
FIG. 2 shows an example of the operation of the driving buffer circuit section in FIG. 1.
When the operation control signal is activated, the driving buffer circuits 71.sub.1 to 71.sub.n amplify the respective bit signals of the parallel display data using the buffers, and then supplies output signals OUT.sub.1 to OUT.sub.n to the output terminals 72.sub.1 to 72.sub.n. The output signals OUT.sub.1 to OUT.sub.n are supplied to a matrix liquid-crystal unit (not shown) as a dynamic driving signal.
As seen from the timing waveform diagram in FIG. 2, however, in the driving buffer circuit section of FIG. 1, the driving buffer circuits 71.sub.1 to 71.sub.n for 64 bits operate simultaneously on the basis of the operation control signal supplied from the control signal line 75.
Therefore, when the level of each of the data inputs to the driving buffer circuits 71.sub.1 to 71.sub.n are switched (to the inverted level), a large switching current will flow because of simultaneous operation of the driving buffer circuits 71.sub.1 to 71.sub.n, thus making switching noise greater.
The switching noise can cause faulty operation of the other circuits in the display data driving IC or of the input circuits of the other ICs connected to the same power line to which the display data driving IC are connected. When the display data driving IC is composed of a CMOS IC, the switching noise acts as the trigger to induce a latch-up phenomenon.
When electronic equipment such as a word processor, a lap-top or notebook personal computer, or a game machine with a liquid-crystal display is provided with the display data driving IC, electromagnetic interference (EMI), part of the specification of the electronic equipment can be induced.
Liquid-crystal display units are now using more and more channels as their screens become larger, and the number of the driving buffer circuits used has almost reached 280. Furthermore, the driving buffer circuits have the function of converting a logic-level signal into a high driving voltage and outputting the high voltage, the absolute value of the amplitude of the recent liquid-crystal driving voltage has reached nearly as high as 40 to 60 V, as compared with a conventional liquid-crystal driving voltage whose absolute value was nearly 20 to 30 V.
Thus, in such a situation, the switching noise tends to become much larger, making the noise problem much more serious.
FIG. 3 shows an example of the display driving signal output driving buffer section in another conventional display data driving IC which are designed to alleviate the problems described above.
FIG. 4 shows an example of the operation of the circuit in FIG. 3.
The driving buffer circuit section in FIG. 3 differs from that of FIG. 1, in that the input signal IN from an operation control signal input terminal 73 is waveform-shaped by inverter circuits 741, 742, and the shaped signal is supplied to a first control signal line 751, that the signal on the first control signal 751 is waveform-shaped by inverter circuits 743, 744 and the shaped signal is supplied to a second control signal line 752, and that the odd-numbered (odd channel) ones of the driving buffer circuits 71.sub.1 to 71.sub.n are all supplied with the operation control signal from the first control signal line 751, and the even-numbered (even channel) driving buffer circuits are all supplied with the operation control signal from the second control signal line 752.
Therefore, because half of the driving buffer circuits 71.sub.1 to 71.sub.n operate with a little different timing from that of the other half by two phases of the operation control signal, even when the level of each of the driving buffer circuits 71.sub.1 to 71.sub.n is switched (to the inverted level), the entire switching current of the driving buffer circuits 71.sub.1 to 71.sub.n is suppressed, thereby minimizing switching noise.
However, because the driving buffer circuit section of FIG. 3 is controlled by two phases of the operation control signal, the output signal group consequently has 10 two phases, thus degrading the display quality of the matrix liquid-crystal display unit driven by the output signal group.
Particularly, when a multi-gradation (i.e., multi-tone) liquid-crystal unit widely used in the recent electronic equipment with liquid-crystal display is driven by the driving buffer circuit of FIG. 3, the phase difference between driving signal groups changes the absolute value of the liquid-crystal applied voltage, making it impossible to obtain the desired tone display.
As described above, the conventional display data driving IC has the problem that simultaneous operation of the channel driving buffer circuits causes a large switching current to flow as a whole, making switching noise larger. Furthermore, when the operation control signal is allowed to have a phase difference to suppress the switching current in the channel driving buffer circuits, this introduces the problem of being unable to obtain the desired multi-tone display when driving a multi-tone display unit.